A grinder, sander, or polisher is a power tool that includes a motor configured to drive rotation of a grinding wheel, a cut-off disc, a sanding disc, a polishing wheel, or like implement. In the present disclosure, the term “grinder” is used to refer to any of the foregoing, or similar, power tools, while the term “grinding wheel” is used to refer to any of the foregoing, or similar, implements. One illustrative type of grinder is an angle grinder, in which a rotor of the motor rotates about an axis that is non-parallel to an axis about which the grinding wheel rotates. For instance, in some angle grinders, the rotor of the motor and the grinding wheel may rotate about axes that are substantially perpendicular to one another. A grinder may include a drive train (including, by way of example, a number of gears) that transfers rotation from the motor to the grinding wheel. An angle grinder typically includes a set of bevel gears that transfers rotation from the axis about which the rotor rotates (or an axis parallel thereto) to the axis about which the grinding wheel rotates (or an axis parallel thereto).
In many grinders, the grinding wheel is constrained to rotate whenever the rotor of the motor rotates, and vice versa. Similarly, components of the drive train may be constrained to rotate with the grinding wheel and/or with the rotor. During the operation of such grinders, if the rotor of the motor or the grinding wheel is forced to come to a sudden stop, this may cause damage to the other components of the grinder (including, for example, the components of the drive train). For instance, in an angle grinder including a pneumatically-driven air motor, a vane of the rotor (or a fragment thereof) may break off when the vane reaches the end of its useful life. If the broken vane, or fragment, is wedged between another vane of the rotor and a cylinder of the air motor, this may cause the air motor to jam and come to a sudden stop, resulting in components of the drive train also being forced to a sudden stop. However, if the air motor has been driving rotation of a grinding wheel with significant mass at significant speed, the inertia of the grinding wheel resist the sudden stop and may thereby cause damage to components of the driven train. Furthermore, such a sudden stop may also tend to loosen the grinding wheel from the grinder.